A quantitative analysis of the degree and extent of coronary arterial narrowing by atherosclerotic plaques in the entire length of each of the four major coronary arteries in 27 necropsy patients with transmural acute myocardial infarction (AMI) is described at necropsy, and the observations in them are compared to those made in 22 control subjects. Of the 1403 five-mm segments examined in the 27 AMI patients (avg 52 per patient), 484 (3%) were 76-100% narrowed in cross-sectional area by atherosclerotic plaques (controls equal 3%); 528 (38%) were 51-75% narrowed (controls equal 25%); 319 (23%) were 26-50% (controls equal 44%) and only 72 segments (5%) were less than 25% narrowed (controls equal 28%).